The present invention relates to a wall structure for a gas turbine engine, particularly for bounding areas of the gas turbine engine containing gases at elevated temperatures, such as combustion chamber walls or afterburner duct walls.
It is known to provide cooling for walls bounding portions of a gas turbine engine which contain gases at elevated temperatures by providing multiple perforations through the wall to enable a thin film of cooling air to be formed on the inside of the wall surface to protect the wall from the effects of the elevated gas temperatures. The known gas turbine engines direct a portion of the oxidizer, typically air, onto the outer wall surface such that it may pass through the wall via a plurality of small orifices formed in the wall to be cooled.
To achieve optimum cooling permeability, the cooling orifices have been arranged in an array illustrated in FIG. 1A. This conventional cooling orifice array forms a heterogeneous and periodic local air flow output at the chamber outlet which depends upon the positions of the orifices which are generally aligned parallel to the longitudinal axis of symmetry and parallel to the air flow through the engine. The maximum output is obviously located downstream of the aligned orifices.
Typically, gas turbine engine combustion chambers and turbojet engine afterburner ducts are formed from several segments of metal sheet having juxtaposed edges which may be rolled and subsequently welded to form the typically annular structure. The segments from which the annular structures are made have sides which are typically cut off at right angles to their upstream and downstream ends thereby forming a welding seam which runs generally perpendicular to the upstream and downstream edges and, hence, generally parallel to the longitudinal axis of the engine and to the flow of cooling air.
During fabrication of the structural segments which are formed with multiple perforations to achieve the necessary cooling, the positioning of the orifices frequently requires that an axial row of orifices are fabricated closely adjacent to the site of the welding seam, thereby degrading the mechanical strength of the completed annular structure. The elimination of the axial row of orifices close to the welding seam causes an adverse wake to be formed in the air flow which will decrease the cooling efficiency of the air flow at the structure outlet.
It is known that the internal temperatures change between the upstream end of a combustion chamber and the downstream outlet end. The walls which define the inner and outer boundaries of an annular combustion chamber typically are formed in a cylindrical configuration for the outer boundary wall and a frustoconical configuration for the inner boundary wall. Obviously, cooling of the walls using multiple perforations must coincide with the different temperatures present in different axial positions within the combustion chamber.